Fledgling magpie injured?

We had 3 fledgling magpies in our garden earlier this week. 2 have disappeared and the 1 left mainly just sits in the garden. If he tries to hop around, he's very wobbly and falls face first after a few steps. The parents are still protecting it (squarking at our cat who we're keeping in) but I haven't seen them feeding it. It doesnt flap its wings but uses them to balance on the ground. Is there anything we can do to help it?